1. INTRODUCTIONMicrostrip antennas (MSAs) are planar and conformal, which generallyyield polarization based on the orientation of planarity of the hostsurface [1].It is difficult to obtain vertical polarization fromhorizontally mounted MSAs. Besides, microstrip antennas are verynarrowband. A practical application like IFF (Identification Friendor Foe) antenna used in military and commercial aircraft, requires anantenna that is conformal, broadband, omni-directional in azimuthplane and vertical polarization with minimum cross polarizationlevels [2, 3]. To avoid aerodynamic drag and to reduce radar crosssection, IFF antenna is made flat with the surface of the fuselage orwings.In order to meet all the above requirements, a novel cavitybacked suspended hexagonal patch antenna has been designed as aconformal antenna, which covers the frequency band of 1 to 1.1 GHz.It has similar characteristics as that of a conventionally used verticalmonopole blade antenna, with omni-directional azimuthal radiationReceived 16 September 2013, Accepted 6 November 2013, Scheduled 9 November 2013* Corresponding author: Kamla Prasan Ray (kpray@rediffmail.com).

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pattern and minimum cross polarization level. It is compact and

can be flush mounted below the skin of the aircraft. It satisfies allrequirements of the application of IFF and can replace conventionallyprotruding blade antennas.2. DESIGN OF ANTENNA CONFIGURATIONCavity backed antennas have been reported to increase gain andbandwidth [46]. A circular cavity enclosed circular patch antennahas been reported to yield horizontal polarization, when mounted ona horizontal plane surface [4]. To obtain vertical polarization and tomake the configuration compact, the patch has been designed withvertical shorting posts. It is envisaged that a slot formed between thepatch and the cavity excites current on vertical shorting posts andwould yield vertical polarization. The suspended configuration hasbeen chosen, as the bandwidth requirement was around 9.52% (1.0 to1.1 GHz). A hexagonal patch antenna approximates the circular onewithout reducing the aperture of the antenna [7]. Here, the hexagonalconfiguration has been chosen over the circular patch for the ease offabrication. Rectangular and triangular patches were not consideredas they have asymmetrical geometry. A single point shorting post onthe periphery reduces the resonance frequency of the circular patchthe maximum, by a factor of , leading to the maximum reductionof size of the antenna [1]. At the same time a single shorting postintroduces high level of asymmetry in the configuration, which leadsto asymmetrical radiation patterns and high cross polarization levels.Thus, three alternate vertices of the hexagonal patch were shorted withstraight shorting posts. Further, it is reported that a circular cavityalso reduces the resonance frequency by about 5%, depending uponthe diameter and the height of the rim of the cavity [4].Keeping all these aspects in mind, an unshorted hexagonal patchwas designed with side length of 4.45 cm and the height of patch fromthe ground plane was 3.0 cm. This patch resonates at 1.578 GHz. Next,this hexagonal patch was shorted to the ground plane by 3 cylindricalmetal posts having height of 3.0 cm and diameter of 0.5 cm each, atthree vertices of the patch as shown in Figure 1(a). The diameter ofthe co-axial feed at the centre of the hexagonal patch was optimizedfor good impedance matching over the entire band. The resonancefrequency of this shorted hexagonal patch reduced to 1.231 GHz from1.578 GHz. Finally, the antenna was placed inside the circular cavity.The diameter of the cavity and the height of the rim were optimisedat 16 cm and 3.5 cm, respectively, to achieve the required resonancefrequency of 1.05 GHz. The height of the patch from ground plane

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Figure 1.Cavity backed shorted hexagonal patch antenna.(a) Schematic diagram of the antenna (all dimensions in cm) and(b) photograph of the fabricated antenna.is quite large, i.e., 3 cm. When an N -type co-axial connector withelongated centre conductor is used to feed the antenna, the inputimpedance becomes highly inductive and the impedance loci curveshifts upward on the Smith Chart and thus VSWR becomes high.The radius of the centre conductor of the connector was optimizedand increased from 0.16 cm to 0.8 cm to neutralize the inductivecomponent in the input impedance and thus, improves input matchingover the bandwidth. The cavity isolates the antenna from otherobjects, when it is mounted below the skin of the aircraft.3. MEASURED AND SIMULATED RESULTSThe fabricated antenna is shown in Figure 1(b). Various simulatedresults, using HFSS software, are compared with measured ones [8].The input VSWR plots and corresponding input impedance loci in theSmith Chart are shown in Figures 2(a) and 2(b), respectively, for therequired frequency range of 1.0 to 1.1 GHz. The VSWR is less than 2over the entire bandwidth, which is 9.52% at the centre frequency of1.05 GHz. There is good agreement between simulated and measuredplots.The simulated current distribution is shown in Figure 3.The simulated and measured Azimuthal and Elevation radiationpatterns, at 1.05 GHz, are compared in Figures 4(a) and 4(b),respectively, which are in good agreement. The azimuthal radiationpattern is omni-directional, whereas in elevation, it is a Figure ofEight with a 3 dB beamwidth of 119.40 . Although the beamwidth inazimuth is similar to that of a dipole, the beamwidth in elevation islarger than that of a dipole. As a result, the antenna gain is around1 dBi which is lesser than that of a dipole over the entire frequency

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Figure 2. VSWR and input impedance. (a) VSWR of the cavity

Figure 3. Simulated current distribution at 1.05 GHz.

range of 1.0 to 1.1 GHz. However, due to this larger coverage, theantenna is suitable for applications like IFF antenna. It is also notedfrom the plots, that the cross polarization levels in desired directionsis more than 25 dB down as compared to the co-polarized levels. Theradiation efficiency of the antenna is close to 100% as it is made up ofcopper and is almost loss-less at the design frequency. This horizontallymounted conformal antenna has all the characteristics similar to thatof a vertically mounted protruding blade antenna, conventionally usedfor airborne IFF transponder application, but without high levels ofwind drag.